1. Field of the Invention
The invention relates to radio frequency communication and more particularly, to variable gain amplifiers used in radio frequency signal transmission systems.
2. Description of Related Art
Variable gain amplifiers (VGAs), devices that attempt to maintain a desired output signal level by adjusting gain, are generally used with radio receivers. As such, the cost of a VGA is significant to the overall cost of the radio receiver function. Currently, most VGAs are fabricated with bipolar processes. Bipolar processes are expensive, costing nearly twice as much as a CMOS fabrication processes. However, little development of VGAs in CMOS has occurred due to noise concerns and due to concerns of integrating VGAs fabricated in CMOS with other radio frequency functions because variation in the input and output impedances of a VGA fabricated with CMOS technology can significantly affect the overall operation of a radio frequency system.
One development of a variable gain CMOS amplifier may be found in U.S. Pat. No. 5,757,230. The variable gain amplifier of the '230 patent uses a linearized transconductor in conjunction with an output circuit. The transconductor has a transconductance that varies in response to a first control signal while the output circuit has a transresistance that varies in response to a second control signal. Both control signals are provided by a gain controller circuit portion. The voltage gain of the amplifier is equal to the product of the transconductance and the transresistance. The amplifier can be configured to provide an exponential gain. The amplifier of the '230 patent is a complicated device requiring as many as 50 transistors to implement.
In view of the above, there is a need for a VGA that can be fabricated with low-cost CMOS technology, provide a desirable signal-to-noise ratio, and maintain predetermined, and constant input and output impedances while providing a straight forward design.